CD40 Is Constitutively Expressed on Platelets and Provides a Novel Mechanism for Platelet Activation

Abstract

Abstract-CD40 is a 48-kDa phosphorylated transmembrane glycoprotein belonging to the TNF receptor superfamily.CD40 has been demonstrated on a range of cell types, and it has an important role in adaptive immunity and inflammation. CD40 has recently been described on platelets but platelet activation by CD40 has not been described.In the present study, we use flow cytometry and immunoblotting to confirm that platelets constitutively express surface CD40. CD40 mRNA was undetectable, suggesting that the protein is synthesized early in platelet differentiation by megakaryocytes. Ligation of platelet CD40 with recombinant soluble CD40L trimer (sCD40LT) caused increased platelet CD62P expression, ␣-granule and dense granule release, and the classical morphological changes associated with platelet activation. CD40 ligation also caused ␤ 3 integrin activation, although this was not accompanied by platelet aggregation. These actions were abrogated by the CD40L blocking antibody TRAP-1 and the CD40 blocking antibodies M2 and M3, showing that activation was mediated by CD40L binding to platelet CD40. ␤ 3 integrin blockade with eptifibatide had no effect, indicating that outside-in signaling via ␣ IIb ␤ 3 was not contributing to these CD40-mediated effects. CD40 ligation led to enhanced platelet-leukocyte adhesion, which is important in the recruitment of leukocytes to sites of thrombosis or inflammation. Our results support a role for CD40-mediated platelet activation in thrombosis, inflammation, and atherosclerosis.

Microparticles in acute coronary syndrome

“…We have previously shown that activated platelets bind to leukocytes and can be detected as platelet monocyte aggregates (PMA) (30). Levels of activated PMA were higher in the ACS group than in the SA group in both the CO (p=0.0003) and RA (p=0.04) Figure 5.…”

New Markers of Inflammation and Endothelial Cell Activation

“…Furthermore, CD40 has recently been shown to be constitutively expressed on the surface of platelets and that its ligation results in platelet activation, which may serve to further enhance platelet-driven thrombus formation. 63 Finally, CD40L inhibits EC migration. 58 By preventing ECs to migrate, reendothelialization of any plaque erosions is impaired, enhancing the possibility of an acute atherosclerotic event.…”

Developmental Changes in Soluble CD40 Ligand

“…When platelets are activated, CD40L moves from intracellular sites to the platelet surface (1,3,4). Membrane CD40L may then be cleaved, producing a soluble trimeric fragment known as sCD40L.…”

“…(3,8,9). CD40L also plays a role in thrombosis by inducing TF expression on endothelial cells and monocytes, operating as a platelet agonist, and stabilizing arterial thrombi (1,3,8,10). Notably, these functions are not mediated exclusively via CD40L-CD40 interactions, because the integrin receptors αIIb βIII and α5β1 have also been found to bind CD40L (10,11).…”

“…Its ligand, CD40L (formally CD154), is a TNF family trimeric transmembrane protein that was initially discovered on cells of the immune system (3,4). Both CD40 and CD40L have been identified on macrophages, endothelial cells, vascular smooth muscle, fibroblasts, and most notably, platelets (3,4,5,6,7).When platelets are activated, CD40L moves from intracellular sites to the platelet surface (1,3,4). Membrane CD40L may then be cleaved, producing a soluble trimeric fragment known as sCD40L.…”